1. Field of the Invention
This invention generally relates to reactive power compensators, and more particularly to reactive power compensators with capacitors.
2. Description of the Prior Art
It is well known that the reactive power caused by a reactive load connected to an AC power supply network can be compensated with capacitors. ASEA Reg. 48976,7143 discloses reactive power compensators having capacitors connected in parallel with the load and in series with a pair of anti-parallel thyristors for each capacitor. The thyristors are switched on and off in accordance with changes of the reactive power of the load.
In the above-cited paper, however, the reactive power is estimated based only on the current in the reactive load, and the current in the compensator itself is not measured or included in such an estimate. Therefore, these reactive power compensators use an open-loop control system, and there will remain some deviation between the ultimate state and the target state, where the reactive power at the feeder for the load and the coupled compensators is minimized.
If the reactive power value used in the compensator control system is estimated based on the current at the feeder for the load and the coupled compensators, the control system would form a closed loop, which would result in more precise control than an open-loop control. However, when the reactive load changes rapidly and the compensation control system responds rapidly, such a closed loop control can cause an oscillatory reaction called "hunting" with a characteristic frequency determined by the power supply system and the control system.